Modern communication systems are more commonly using linear modulation techniques due to the high spectral efficiency these techniques provide. A tunable reference oscillator is commonly used to provide a frequency reference for use in demodulating a received signal that was modulated with a linear modulation technique. The tunable reference oscillator normally provides the reference frequency for a frequency synthesizer, which in turn is tuned to the expected carrier frequency of a signal to be received. For simplicity of discussion, the reference oscillator/synthesizer combination can be viewed as a single oscillator that is referred to as a variable reference oscillator. The output of the variable reference oscillator is input to a receiver. The receiver then demodulates the received signal into a voice or data signal using the frequency signal from the variable reference oscillator.
When a communication unit is first powered up, the unit must find the appropriate frequencies on which it is to transmit and receive. This process can be long and difficult, especially if the variable reference oscillator is not in close frequency synchronization (sync) with the actual carrier frequency of the received signal. Further, if a large number of channels must be scanned, existing sync techniques are far too time consuming and require too large an amount of time to accomplish registration on the radio system within a reasonable time. As a further complication, in some situations, sync must be obtained without decoding the received information.
In order to keep a receiver in tune with the carrier frequency of a transmitted signal, various automatic frequency control (AFC) techniques are employed. In such tuning, various adjustments are made to the variable reference oscillator to keep it in tune with the carrier frequency of the transmitted signal, as that frequency may vary or fluctuate for numerous reasons. The frequency difference between the frequency of the variable reference oscillator and the carrier frequency is referred to as frequency error. These adjustments are often referred to as frequency offset adjustments or simply frequency offsets (f.sub.o) and are used to correct frequency errors.
A potential problem with AFC techniques is the lack of precise knowledge of the tuning sensitivity of the reference oscillator. Typically, this sensitivity function is highly non-linear and varies with temperature. Hence, when a variable reference oscillator is adjusted, it is not known how accurately or to what degree the reference will be adjusted. If the adjusted frequency was not within approximately a narrow band, e.g., around 500 Hz or so, a narrowband sync correlation technique, as is known in the art, may miss the ability to synchronize after frequency tuning and lose the channel.
Typically, existing high capacity linear coherent demodulation systems are operational over only a very small bandwidth (.+-.350 Hz for demodulation, .+-.500 Hz for reliable narrowband sync detect). The problem where the change in frequency may be greater than that which can be accommodated by a narrowband sync and/or demodulation algorithms is not as critical in existing FM (Frequency Modulation) carrier technologies due to the inherent ability of FM demodulators to operate in the context of larger frequency offsets.
Accordingly, there is a desire for the ability to accurately estimate the frequency offset of a received message.